1. Field of the Invention
The present invention relates to a method and apparatus for demodulating the read-out signal of a fiber gyroscope. More particularly, the invention pertains to demodulation of a fiber-optical Sagnac interferometer for measuring rotation rate.
2. Description of the Prior Art
Previous demodulation methods for the reading out the signal from a fiber optic gyroscope have exhibited a number of technical problems in practice. Such problems have been particularly evident in non-restored gyroscopes.
The most frequent readout technique by far for fiber-optic gyroscopes (both interferometer and resonator), phase or frequency modulates the light in the gyroscope by means of a modulator. The optoelectrical output signal(s) is (are) then synchronously demodulated, (i.e., multiplied by a signal having a fixed phase relationship to the modulation signal) and then lowpassfiltered. The signal thus obtained provides a measure of the rate of rotation.
The dynamic range required for the demodulator (particularly in the case of non-restoring gyroscopes) is extremely high, often exceeding six decades. The commonly required high frequency of 100 kHz raises extremely difficult technical problems in the construction of a demodulator meeting such requirements.
A number of other problems, hitherto unsolved, or requiring extremely high technical expenditures, are also encountered in this area. These include:
(1) The gyroscope modulation or its harmonics (e.g., from a digital drive) reach the input of the demodulator by induction and appear as offset errors due to the synchronous demodulation. PA1 (2) Fast synchronous demodulators are essentially constructed of FET switches. Accordingly, their precision is effected by error terms caused by charge carrier injection during the switching processes. PA1 (3) In square wave modulation and demodulation, a high degree of sensitivity to changes in the phase difference between the modulation signal and the signal supplied to the demodulator as a reference occurs. PA1 (4) In practice, digital synchronous demodulation, (i.e., the reading-in of the rate-of-rotation dependent signal of the fiber-optic gyroscope, PA1 (5) In most cases, an amplitude modulation (lost modulation) is associated with a phase or frequency modulation of the light signal in the fiber-optic gyroscope. This may limit or gate out parts of the photodetector signal. Unfortunately, the electronic gating circuits can introduce errors due to charge carrier injection or unsatisfactory gating.
obtained via a photodetector with high repetition rate and subsequent digital synchronous demodulation) cannot be achieved with high accuracy in the case of non-restored gyroscope arrangements. This results from the fact that A/D converters are required that must be very fast and have high resolution and linearity due to the wide dynamic range. Such A/D converters are not presently available.